BY RAMU SRIKAKULAPU ASSISTANT PROFESSOR SHARDA UNIVERSITY

UNIT -1
INTRODUCTION TO POWER SYSTEMS
Need

for protection Nature and causes of faults Types of faults Fault current calculation using symmetrical components Power system earthing Zones of protection Primary and back up protection Essential qualities of protection Typical protection schemes.

TEXT BOOKS
Badri ram , D.N.Vishwakarma, power system protection & switchgear , Tata McGraw hill publishing company ltd, New Delhi. C.L Wadhwa , electrical power systems , New Age International (p) limited. B. Ravindranath, and N. Chander, Power System Protection & Switchgear, Wiley Eastern Ltd., 1977. Sunil S. Rao, Switchgear and Protection, Khanna publishers, New Delhi, 1986 .

What is the power system ?

POWER FLOW

NEED FOR PROTECTION

Heavy currents associated with short circuits is to cause for damage If short circuit presents on a system for longer time , it cause to damage the some of sections in system

Fire exists System voltage reduces to low level and generators in power station may lose synchronism By un cleared fault to cause for failure of the system

To protect the system from faults need to use automatic protective devices like

Circuit breakers Protection relays ,isolate the faulty element

PROTECTION SCHEME OPERATION

C.B can disconnect the faulty element of the system Protective relay is to detect and locate a fault and trip C.B Protective relay senses abnormal conditions in system and gives alarm signal Those abnormal conditions are

short circuits Over speed of generators & motors Over voltage Under frequency Loss of excitation Over heating of stator& rotor of an alternator

CAUSES OF FAULTS

Causes of fault on over head lines

Birds bodies touch the phases Direct lighting strokes Air crafts Snakes Ice Snow loading Earth quakes

Causes of faults in case of cables, transformers, generators

Failure of solid insulation Flash over due to over voltages

DIRECT LIGHTING STROKES

AVERAGE 400 KV LINE FAULT FREQUENCY STATISTICS BY FAULT CAUSE PER 100 KM PER YEAR

TRANSMISSION LINE FAULT CAUSES 12,229 FAULTS FROM 132 KV TO 765 KV

TYPES OF FAULTS

Unsymmetrical faults
Single phase to ground fault(L-G) Line to line fault(L-L) Double line to ground fault(L-L-G) Open circuited phases fault Winding faults

Symmetrical faults
3phases to ground fault(LLLG) 3phase fault

SYMMETRICAL FAULTS
All 3 phases are short circuited In case of symmetrical fault current is equally shared by the 3phases In symmetrical to analysis fault by one phase only Only +Ve sequence component exist in symmetrical Remaining two components always zero.

OPEN CIRCUITED PHASES FAULT

Caused by a break in the conducting path Occurs when

one or more phase conductors break cable joint or a joint on the over head lines fails

Fault rises when C.Bs or isolators open but fail to close one or more phases

WINDING FAULTS

Occurs on the alternator, motor & transformer windings

SIMULTANEOUS
FAULTS

PERCENTAGES OF

FAULT IN POWER SYSTEM 70-80% 17-10%

L-G L-L

LL-G
3 PHASE

10-8%
3-2%

EFFECTS OF FAULTS
Heavy short circuit current cause to damage to equipment Arcs associated with short circuits may cause fire Reduction in the supply voltage of the healthy feeders Loss of industrial loads Heating rotating machines due to reduction in the supply V&I Loss of stability

ZONES OF PROTECTION
Separate protective schemes for each piece of equipment or element of the power system. dependence on the equipment divided in to a number of the zones Each zone covers one or a the most two elements of the power systems The protective zone plan to cover the entire power system. Adjacent protective zones must over lap each other A relatively low extent of over lap reduces the probability of faults in that region. Tripping of too many breakers does not occurs frequently.

EXAMPLE

PRIMARY & BACK UP PROTECTION

Every zone have a suitable protection scheme If fault occurs in a particular zone, it is duty of primary relays of the zone to isolate the faulty element. If its fails, back up protection scheme is to clear the fault. The protection schemes improves the system performance

BACK UP RELAY
It operates after a time delay to give the primary relay sufficient time to operate. When a back up relay operates a larger time of the power system is disconnected from the power sources. Types of back up relays are

Remote back up Relay back up Breaker back up

REMOTE BACK UP

It is cheapest and simplest

It is widely used for back up protection of transmission lines Most desirable It will not fail due to the factors causing the failure of the primary protection

RELAY BACK UP

Additional relay is provided for protection. It trips the same CB if the primary relay fails and this operation takes place with out delay. Costly Used where back up is not possible.

BREAKER BACK UP
Bus bar fault : When protective relay operates in response to a fault but the C.B fails to trip, the fault is treated as a bus bar fault. used for bus bar system , where a number of C.BS are connected to it. At time of bus bar fault , necessary that all other C.Bs on the bus bar should trip.

BACK UP PROTECTION
Breaker 5 Fails C A D E

2 T

11

12

10

QUALITIES OF PROTECTION

The basic requirements of a protective system are

Selectivity or discrimination Reliability Sensitivity Stability Fast operation

SELECTIVITY
The selectivity of protective system dependence on quality of a protective relay It is able to discriminate between in the protected section & normal section. It should be able to distinguish whether a fault lies with in its zones of protection or outside the zone. The relay able to discriminate between a fault & transient conditions

power surges inrush of a transformers magnetizing current.

INRUSH OF A TRANSFORMERS MAGNETIZING

CURRENT When a transformer is first energized, a transient current up to 10 to 50 times larger than the rated transformer current can flow for several cycles inrush happens when the primary winding is connected at an instant around the zero-crossing of the primary voltage (which for a pure inductance would be the current maximum in the AC cycle). In the absence of any magnetic remanance from a preceding half cycle, the effective magnetizing force is doubled compared to the steady state condition

RELIABILITY
The typical value of reliability of protective system is 95% reliability dependence on

Robustness Simplicity In case of relay Contact pressure Contact material of relay

To achieve a high degree of reliability

Design Installation Maintenance Testing of the various element of the protective system.

SENSITIVITY
It dependences on pick up current value Pick up current:a protective relay should operate when the magnitude of the current exceeds the present value.

A relay should be sensitive to operate when the operating current just exceeds its pick up value.

STABILITY

Protective system should stable for

Large current due to fault Internal fault External fault

FAST OPERATION
It means Isolate the faulty element quickly. To minimize damage to the equipment . To maintain system stability To avoid the loss of synchronism

The operating time of the protective system should not exceed the critical clearing time. protective relay to clear the fault.

Critical clearing time:- the minimum time taken by

By fast operation of protection system to avoid

Burning due to heavy fault current Interruption of supply to consumers Fall in system voltage it results Loss of industrial loads

Operating time is usually in one cycle or half cycle.

PROTECTIVE SCHEME CLASSIFICATIONS

A protective scheme is used to protect an equipment or a section of the line The protective schemes are Over current protection Distance protection Carrier current protection Differential protection

OVER CURRENT PROTECTION

This protection includes one or more over current relays. over current relay operates when the current exceeds its pick up value. It is used for the protection of

Distribution lines Large motors

DISTANCE PROTECTION
It includes number of distance relays of same or different. Distance relay measures the distance between the relay location and the point of fault in terms of impedance, reactance Relay Type Measurement
1. 2. 3.

Impedance relay Reactance relay Mho relay

impedance reactance admittance

This protection scheme is used for the protection of

transmission lines Sub- transmission lines

CARRIER CURRENT PROTECTION

In this used relays are distance type. Their tripping operation is controlled by the carrier signal. A transmitter and receiver are installed at each end of transmission line to be protected. Information of direction of fault current is transmitted from one end of the line section to the other by carrier signal. Relays placed at end It trips if the fault lies with in their protected section. Do not trip for external fault. It is used for the protection of EHV &UHV line (132kv above)

DIFFERENTIAL PROTECTION

C.TS are placed on both side of each windings of a machine. The outputs of their secondaries are applied to the relay coils. The relay compare the current entering & leaving of a machine winding This difference in the current actuates the relay. In case of internal fault , the current values are not equal. The relay inoperative under normal condition and external fault. It is used for the protection of
Generators Transformers Motors of various size Bus zones

In bus zone protection, C.TS are placed on the both sides of the bus bar.

FAULT CURRENT
CALCULATION

SEQUENCE COMPONENTS

UNSYMMETRICAL VECTOR FROM

SYMMETRICAL COMPONENTS

Relations of voltage components in matrix form

Relations of current components in matrix form

SYMMETRICAL VECTOR
FROM UNSYMMETRICAL VECTOR

SEQUENCE IMPEDANCE

SYMMETRICAL COMPONENTS OF
GENERATOR

POSITIVE SEQUENCE

IMPEDANCE

NEGATIVE

SEQUENCE IMPEDANCE

ZERO SEQUENCE IMPEDANCE

SYMMETRICAL COMPONENTS OF TRANSFORMER (ZERO SEQUENCE)

ZERO SEQUENCE IMPEDANCE OF 3 LOAD

No grounding Solid grounding Neutral impedance grounding delta

L-G FAULT

L-L FAULT

LL-G FAULT

NEUTRAL GROUNDING
The performance of the system in terms of the short circuits, protection etc. is greatly affected by the state of the neutral. There are various method of grounding the neutral of the system.

Solid grounding Resistance grounding Reactance grounding Voltage transformer grounding Zig-zag transformer grounding

UNGROUNDED SYSTEM
In

case of L-G fault at phase C charging current = 3*(per phase charging current) The voltages of the healthy phases rises to 0.866Vph These voltages can be eliminated by connecting an inductance of suitable value between the neutral and the ground or Arcing ground.

RESONANT GROUNDING
The value of the inductive reactance is such that the fault current balances exactly the charging current . Resonant grounding is also called ground fault neutralizer or Peterson coil.

The resonant grounding will reduce the line interruption due to transient line to ground fault.

SOLID GROUNDING

The neutral is connected directly to the ground with out any intentional impendence between neutral and the ground.

For low voltages up to 600volts and above 33 kV.

RESISTANCE GROUNDING
The value of the resistance commonly used is quite high (in order to limit power loss in resistor during LG-fault) as compared with the system reactance. It is normally used where the charging current is small i.e., for low voltage short length over head lines. It reduces the ground hazards. It has helped in improving the stability of the system during ground fault by replacing the power dropped as a result of low voltage, with an approximating equal power loss in the resistor. To limit the stator short current . Generators are normally provide with resistance grounding.

REACTANCE GROUNDING

Reactance grounding system (Xo/X1) >3 Solid grounding system (Xo/X1) < 3

Reactance grounding may be used for grounding the neutral of synchronous motors & synchronous capacitors and also for circuits having large charging current
For medium voltages 3.3kV & 33KV resistance or reactance grounding is used.

ZIG-ZAG TRANSFORMER GROUNDING

It is used , if neutral point is requied which other wise is not available (eg. Delta connection, bus bar points)

ADVANTAGES OF NEUTRAL
GROUNDING

Voltages of the phases are limited to phase to ground voltages The high voltages due to arcing grounding or transient line to ground faults are eliminated The over voltages due to lighting are discharged to ground

ADVANTAGES OF ISOLATED NEUTRAL

It is possible o maintain the supply with a fault on one line Interference with communication lines is reduced because of the absence of Zero sequence currents.